Radiation therapy for cancer treatment has been in use for several decades. Modern radiation therapy systems typically generate high intensity x-rays by bombarding a suitable target with high energy electrons. X-rays are emitted from the target in a generally conical pattern and are initially confined to a generally rectangular beam by moveable, x-ray blocking “jaws” in the head of the system. Typically, the patient is positioned about 1 meter from the x-ray target and, when fully open, the jaws define a square treatment area that is about 40 cm×40 cm at the patient plane. However, in many instances it is important to irradiate only a precisely defined volume conforming to a tumor, thus the target site must be irradiated from multiple angles. Rarely, however, can the system jaws alone be used to implement a suitable treatment plan. While the use of x-rays is the predominant technique for radiation therapy, high energy particles, such as electrons and protons, are also sometimes used. Accordingly, as used herein the term radiation therapy is intended to encompass all such techniques, and the present invention has application to all such techniques. Thus, when reference is made herein to x-rays or radiation, such terms should be also understood to encompass high energy particles.
Multi-leaf collimators (MLCs), such as described in the co-assigned U.S. Pat. No. 4,868,843, issued Sep. 19, 1989, to Nunan, (the disclosure of which is incorporated by reference), have been almost universally adopted to facilitate shaping of the radiation beam so that the beam conforms to the site being treated, i.e., the leaves are adjusted so that the beam conforms to the shape of the tumor from the angle of irradiation. Subsequent to its introduction, the MLC has also been used to perform a technique known as “Intensity Modulated Radiotherapy” (IMRT), which allows control over the radiation doses delivered to specific portions of the site being treated. In particular, IMRT allows the intensity distribution of the radiation reaching the patient to have almost any arbitrary distribution. IMRT can be implemented by iteratively positioning the leaves of the MLC, which form an aperture through which radiation is delivered, to provide desired field shapes which collectively deliver the desired dose distribution. IMRT techniques can either be static (“point and shoot”), in the sense that the leaves do not move when the beam is on or, alternatively, as in systems sold by the assignee of the present invention, be implemented using a “sliding window” approach, in which the leaves of the MLC are moved continuously when the beam is on. IMRT is typically implemented by using an elongated aperture or window that is oriented perpendicular to the direction of leaf motion, as depicted in FIG. 5. Specifically, in sliding window IMRT the overall speed of leaf motion and the separation of leaf pairs are independently adjusted as the window moves, such that different portions of the treatment field are irradiated with different doses of radiation through an aperture that changes shape as it is being moved.
Radiation therapy is generally implemented in accordance with a treatment plan which typically takes into account the desired dose of radiation that is prescribed to be delivered to the tumor, as well as the maximum dose of radiation which can be delivered to surrounding tissue. Various techniques for developing treatment plans are well known. Preferably, the computer system used to develop the treatment plan provides an output that can be used to control the radiation therapy system, including the MLC leaf movements. Typically, the desired dose prescribed in a treatment plan is delivered over several sessions, called fractions.
Tumors and surrounding tissue, including critical organs, may move in a periodic fashion while a site is being irradiated, for example, as a result of normal respiratory motion. (As used herein “periodic” is meant to have a broad meaning and includes any repeated motion, such as breathing motion, even if irregular.) Heretofore, no effort has been made to take such movement into account when developing a treatment plan and, therefore, movement in the treatment field can have a significant impact on the effectiveness of a treatment plan. A treatment plan that does not take such movement into account may result too much or too little radiation reaching the intended target region and/or too much radiation reaches surrounding tissue. The extent of the problem caused by the mismatch varies, and can range, in extreme cases, from very little radiation delivered to the target to a delivery of several times the intended dose. Other types of deviations from the prescribed radiation delivery may occur, causing additional problems with the effectiveness of the treatment plan.
The quantity of incident radiation, or fluence, delivered is a sum of the radiation allowed through the aperture over the course of the exposure. In the worst case scenario, the target region may receive several times the prescribed dose when the target region movement is in phase with the aperture movement. On the other hand, if movement of the target region is out of phase with the window movement, the tumor may receive a lower than prescribed dose, or no dose. In practice, interplay between these movements has been reported to generate differences of greater than 10% between the delivered and the planned dose distributions for a single fraction. Obtaining the desired biological response in the target region depends upon delivery of the intended fractional dose, thus achieving the planned dose distribution is critical to success of the treatment.